Nematic liquid crystal boojums with handles on colloidal handlebodies

TL;DR

This study demonstrates how the surface topology of colloidal particles influences the formation and structure of boojum defects in nematic liquid crystals, revealing new ways to control defect arrangements for advanced material design.

Contribution

It introduces a method to generate and analyze boojums on handlebody-shaped colloids with different genus, linking surface topology to defect structure and stability.

Findings

Handlebody-shaped particles induce at least 2g - 2 boojums, conserving topological charge.

3D imaging reveals complex, handle-shaped defect cores.

Numerical modeling matches experimental director fields and defect structures.

Abstract

Topological defects that form on surfaces of ordered media, dubbed boojums, are ubiquitous in superfluids, liquid crystals (LCs), Langmuir monolayers, and Bose-Einstein condensates. They determine supercurrents in superfluids, impinge on electrooptical switching in polymer-dispersed LCs, and mediate chemical response at nematic-isotropic fluid interfaces, but the role of surface topology in the appearance, stability, and core structure of these defects remains poorly understood. Here, we demonstrate robust generation of boojums by controlling surface topology of colloidal particles that impose tangential boundary conditions for the alignment of LC molecules. To do this, we design handlebody-shaped polymer particles with different genus g. When introduced into a nematic LC, these particles distort the nematic molecular alignment field while obeying topological constraints and induce at least 2g - 2 boojums that allow for topological charge conservation. We characterize 3D textures of boojums using polarized nonlinear optical imaging of molecular alignment and explain our findings by invoking symmetry considerations and numerical modeling of experiment-matching director fields, order parameter variations, and nontrivial handle-shaped core structure of defects. Finally, we discuss how this interplay between the topologies of colloidal surfaces and boojums may lead to controlled self-assembly of colloidal particles in nematic and paranematic hosts, which, in turn, may enable reconfigurable topological composites.